Commuters have been forced to stand in packed trains. One passenger reportedly collapsed due to overloading.

"Just watched a young lady collapse on the train because of the overloading!! Not cool!!" wrote Twitter user @redmandymoo.

Some slammed the Rail and Maritime Transport Union for the delays and "dangerous" conditions.

"The trains were so packed today it was beyond dangerous and it's YOUR FAULT!!!!" wrote @apausina.

"Train carnage this morning… Not sure those striking will win many friends but massive inconvenience," said @Sharks771.

Others blamed Auckland Transport.

"Well done! I cannot imagine how you can get more people from cars to public transport," said @JiriDolezalek.

"Taking the train on the first is already a nightmare and I believe the public deserves better than weeks of misery caused by your actions," said @allenswhuang.

"Sort it out. The rest of us need to make a living too!" said @Pappsy.

A spokesperson for Auckland Transport said that standing on a train is "part of living in a big city".

"There are services that are very busy at the moment," a spokesperson told Newshub.

"It's perfectly safe for people to be standing on a train."

Rail and Maritime Transport Union organiser John Kerr says the strike is about safety.

"Auckland Transport and Transdev want to take train managers off the trains and our members are saying that's going to compromise public safety and they're not prepared to do that.

"[Train managers] are first responders in the event of an emergency, they do crowd management and they're also operational, so if there's a signals failure they intervene. They're really a critical member of staff."